The present invention relates generally to the field of acoustic wave detection and in particular to a new and useful microelectromechanical sensor using cantilever capacitance or piezoresistance to indicate the presence of acoustic wave energy. In particular, the device can be used to sense acoustic signals propagating through the earth, including reflected signals from buried objects having different densities from the surrounding soil.
Microelectromechanical sensors are known for use in sensing the presence or strength of magnetic fields, among other things. The sensors use a differential or change in electrical capacitance or piezoresistance to measure magnetic fields they are exposed to, or measure the magnetic properties of sample crystals in contact with the sensor exposed to a known magnetic field.
Recent developments in acoustics indicate that some types of acoustic signals called solitons can be made to propagate through the earth with little distortion. See, Surajit Sen et al., "Soliton-like Pulses in Perturbed and Driven Hertzian Chains and their Possible Applications in Detecting Buried Impurities," Physica Review E, Vol. 57, No. Feb. 2, 1998; Ellen Goldbaum, "UB method may help detect land mines" UB Reporter, Dec. 4, 1997 at p.2. While this work is primarily theoretical, there is support for the concept and physical experiments have indicated that solitons exist.
Sensors having the sensitivity necessary to detect acoustic signals reflected from small buried objects have heretofore been unknown.
The ability to sense any type of acoustic signals reflected from objects buried in dirt, soil or sand is particularly useful for the task of finding and removing land mines. Land mines are commonly made cheaply and inexpensively from non-metallic materials which cannot be easily detected using current technology. They are typically buried less than a few feet below the surface, however, so that a person may detonate the mine by stepping on the ground above it.
Common methods for eliminating land mines from large areas include bombing the region suspected of having land mines to detonate the buried mines, or, when available, using maps indicating the mine locations to find and remove them. Neither method is particularly reliable and subject to error, either by failing to detonate a mine, or through differences between the map location and the actual mine position.
Thus, a relatively inexpensive, portable sensor system for detecting at least non-metallic objects buried in close proximity to the surface of the earth is desirable.